Plants respond to their changing light environment using three major classes of photoreceptors (phytochromes, blue/UV-A receptors and UV-B receptors). However, relatively little is known about how the light signal is transduced in the cell to provoke various physiological responses. The long term objective of the proposed research is to understand the mechanisms of intracellular light signaling. A novel mutation specific for phytochrome A signaling in Arabidopsis, designated as fin219, has been identified in the host lab. fin219 also confers a dominant suppression of the temperature sensitive phenotype of weak cop1-6 allele in an allele-specific manner, suggesting FIN219 acts as a bridging molecule between phyA and COP1. FIN219 encodes a product similar to a rapidly auxin-inducible protein GH3, implying it might be a potential regulatory target for the plant hormone auxin. The specific aims of this proposal are: 1). Characterize the expression pattern of FIN219 and its regulation by light and auxin; 2). Biochemical and cellular studies of FIN219; 3). Overexpression studies of FIN219 to substantiate its role in phyA signaling; 4). Identify and characterize FIN219 interactive partners.